When creating a new notification, the system does not separate Leave from Sickness. The option in the Table field is Absence (Employment) and this will trigger an email for any absences, regardless if they are annual leave or sickness.
However, once Absence (Employment) is selected in the Table dropdown, you can select sickness related options in the Field dropdown to ensure that the notification only gets triggered by sickness.
For example, you can set up a notification based on the expected return date, as this is a field available only for sicknesses.
To do this:
Go to Setup and click Settings.
Click Notifications, then Add Notification.
Select Absence (Employment) from the Table dropdown.
Select ExpectedReturn from the Field dropdown.
Set up the notification as needed.
Click Save.
This email will then be sent based on the date entered in the Expected Last Day field on an employee's sickness.